Zinc bromide is a widely used compound in chemical, petrochemical and process industries, photography, and manufacturing of synthetics such as rayon. Because commercial usage of zinc bromide is increasing, large-scale production of the compound is necessary.
The reaction of metallic zinc with halides to produce a zinc halide was reported over 150 years ago. Wahl et al., U.S. Pat. No. 220,449, dated Oct. 7, 1879, disclose production of impure zinc chloride by dropping "dross", which contains zinc-iron alloy resulting from galvanizing iron, into aqueous muriatic acid (hydrochloric acid). These early reactions were reported to be sluggish, presumably owing to layers of zinc oxide on the surface. More recently, zinc bromide was prepared by the addition of moist bromine to hot zinc as disclosed in R. Pajeau, Bull Chim. Soc. Fr. [5], 6, 1187-91 (1939) and by the reaction of bromine vapor in a stream of nitrogen with molten zinc disclosed in K. Niwa, J. Fac. Sci. Hokkaido Univ. [1], 3 (1), 17-33 (1940). These reactions did not include solvent, were conducted at high temperatures and were accompanied by the presence of excess quantities of free bromine. Such reactions are impractical for large-scale production of zinc bromide, especially from a health and environmental standpoint. Bromine is an extremely toxic and corrosive chemical, especially in its free form as a liquid or as a gas (boiling point 59.degree. C.).
A. Raynaud, Bull. Chim. Soc. Fr., [4], 39, 195-201 (1926) teaches that bromine and zinc do not react with each other directly and require the formation of the intermediates ZnO and HBr.
The process of preparing metal bromides from free metals and bromine has been reported in patent literature. For example, U.S. Pat. No. 4,138,354 to Sochol et al. discloses a process for preparing manganese and cobalt bromide from manganese, cobalt bromide, and bromine. The process depends upon an oxidation-reduction reaction between manganese and the cobaltous ion and the subsequent reaction of the generated cobalt with aqueous bromine. U.S. Pat. No. 4,248,850 to Keblys teaches the preparation of metal bromides from a basic metal compound and bromine in the presence of formaldehyde as a reducing agent. U.S. Pat. No. 4,514,374 to Kirsch teaches the use of a lower alkanol as a reducing agent.
U.S. Pat. No. 1,483,852 to Rosenstein discloses the production of zinc chloride by reacting lead-zinc alloy in water with chlorine gas. Lead metal is alloyed with the zinc metal, and the lead serves as a catalyst for the reaction of the chlorine with the zinc. Chlorine reacts with the zinc-lead alloy to produce zinc chloride. Heat from the reaction causes water to evaporate as steam. The concentration of zinc chloride is controlled by the rate of water dropping or flowing through zinc particles in a zinc-receiving pipe. The Rosenstein disclosure states that the process cannot be worked with pure zinc.
U.S. Pat. No. 2,415,346 to Farr discloses a process of making halogen salts of metals. Zinc bromide is made by reacting zinc chloride with hydrobromic acid under boiling temperature conditions. In the '346 process of making metal bromides, a metal salt of the type MY.sub.n is reacted with an aqueous medium of hydrogen bromide, where M is the desired metal, Y is a halogen of lower atomic weight than bromine, and n is the valence of M. The metal salt is treated with a substantially equivalent quantity of the acid in an aqueous solution with sufficient water to form a constant boiling mixture.
U.S. Pat. No. 3,723,075 to Stevenson et al., discloses that aluminum tribromide is made by reacting metallic aluminum with bromine in a stirred liquid reaction medium consisting essentially of molten aluminum tribromide at temperatures of 100.degree. C. to 140.degree. C.
None of the references teach or suggest a method for manufacturing zinc bromide from metallic zinc and bromine solution on a large scale, commercial basis. Consequently, there remains a need for an inexpensive, and simple method for rapidly manufacturing zinc bromide in large quantities. The problem of zinc bromide production without the harmful production of free gaseous bromine and hydrogen must also be addressed.